My idea is to make the two columns as similar as possible. So exploiting the parallelism of AC and FD, I found that b°=c°, e°=180°-b°-d° and that f = 180°-(180°-b)-a =b-a. Substituting these equalities in column B, I get that it is equal to 90°+a°-c°+d°. Thus, the answer is C.